What matters is not the volume but the cost. A back-of-the-envolope calculation shows that olivine rock is about 20 times cheaper than crude oil.

Olivine rock: 10-25$/ton Crude oil: $60/barrel, so about $400/ton

Their website reads that "A volume of 7 cubic miles (11 km^3) of olivine, or around 30 Gigatons, is needed each year. This is less than half the volume of construction materials and less than that of fossil fuel equivalents mined yearly".

If olivine rock needs to be mined in comparable amount to fossil fuels to offset global CO2 production, it will cost about 5% of the fossil fuels, i.e., the price of oil will grow by about 5% globally, which is not much.